The present invention relates to a "passive interrogator label system" (PILS); that is a system comprising an interrogator for transmitting an interrogation signal, one or more "labels" or passive transponders which produce a replay signal containing coded information in response to the interrogation signal, and a receiver and decoding system for receiving the reply signal and decoding the information contained therein.
A passive interrogator label system of the type to which the present invention relates is disclosed in the U.S. Pat. No. 3,273,146 to Horwitz, Jr.; U.S. Pat. No. 3,706,094 to Cole and Vaughan; U.S. Pat. No. 3,755,803 to Cole and Vaughan; and U.S. Pat. No. 4,058,217 to Vaughan and Cole. In its simplest form, the systems disclosed in these patents include a radio frequency transmitter capable of transmitting RF pulses of electromagnetic energy. These pulses are received at the antenna of a passive transponder and applied to a piezoelectric "launch" transducer adapted to convert the electrical energy received from the antenna into acoustic wave energy in the piezoelectric material. Upon receipt of a pulse, an acoustic wave is generated within the piezoelectric material and transmitted along a defined acoustic path. Further "tap" transducers or reflectors arranged at prescribed, spaced intervals along this path, respectively, either (1) convert the acoustic wave back into electric energy or (2) reflect the acoustic wave back to the launch transducer for reconversion into electrical energy by the launch transducer. The presence or absence of tap transducers or reflectors at the prescribed locations along the acoustic wave path determines whether a reply pulse will be transmitted with a particular time delay, in response to an interrogation pulse. This determines the informational code contained in the transponder reply.
When an acoustic wave pulse is reconverted into an electrical signal it is supplied to an antenna on the transponder and transmitted as RF electromagnetic energy. This energy is received at a receiver and decoder, preferably at the same location as the interrogator transmitter, and the information contained in this response to an interrogation is decoded.
The transponder in a passive interrogator label system thus operates in the "time domain" to produce a reply signal containing one or more pulses of RF electromagnetic energy in response to the receipt of each single burst or pulse of RF electromagnetic energy. The presence and timing of the reply pulses relative to the transmitted, interrogation pulse determines the informational code contained in the reply.
Passive interrogator label systems of the above described type are subject to a number of disadvantages. The signal to noise performance of these systems is not easily improved since the systems are subject to both wide and narrow band interference. In addition, the systems require extensive signal processing of the transponder reply signal to evaluate and determine the informational code. This signal processing must normally be accomplished at the site of the receiver because the information is contained in a relatively wideband signal. Transmission to a remote, signal processing location would require wideband signal transmission.